The present invention relates to a method of generating image scanning clock signals in an optical scanning device.
There have been known optical scanning devices in which a light beam is cyclically deflected by a rotating light deflector such as a rotating polygonal mirror or a hologram grating disc for writing information on or reading recorded information from an object such as an information storage medium.
When the light beam such as a laser beam is deflected by the rotating light deflector into a scanning beam, the scanning beam is focused as a spot on the information storage medium, which may be an original document carrying information to be read out or a photoconductive photosensitive body on which information is to be written or recorded. While the information storage medium is being scanned with the scanning beam spot, the information is read from or written on the information storage medium by the scanning beam spot modulated by an image scanning clock signal at the rate of one pixel per clock pulse.
The scanning of the information storage medium with the scanning beam is referred to as "primary scanning". While the information storage medium is subject to the primary scanning, it is fed in a direction normal to the direction of the primary scanning. This feeding movement of the information storage medium is referred to as "secondary scanning".
As can readily be understood, positions where respective primary scanning cycles are started should be aligned with each other in the secondary scanning direction in order to write and read information properly. If such starting positions for the primary scanning were not aligned in the secondary scanning direction, then an image reconstructed from the read-out signals would be distorted in the readout mode, or an image signal error known as "jitter" would be produced in the write mode.
It has been general practice to align the primary scanning starting positions in the secondary scanning direction by positioning a light sensor outside of the primary scanning region, detecting the scanning beam prior to each of the primary scanning cycles, and synchronizing the primary scanning cycles.
One way of bringing the primary scanning cycles into synchronism is to use an image scanning clock signal. More specifically, the instant the scanning beam is detected by the light sensor, the pulses of the image scanning clock signal start being counted up to a preset number m, for example, and the primary scanning cycle is started from a position in which the pulse number m+1 is reached. The image scanning clock signal is continuously generated at all times. The output signal from the light sensor, which serves as a reference for synchronizing the primary scanning cycles, is subject to variations in signal interval due for example to different mechanicam accuracies of the rotating light deflector. Dependent on whether the light sensor output signal is generated when the clock signal is high or low, the time to start the primary scanning cycle can deviate at most one clock pulse from the detection of the scanning beam with the light sensor, resulting in a scanning error.
When the time to start the primary scanning cycle deviates at most one clock pulse from the detection of the scanning beam with the light sensor, the primary scanning starting position varies at most one pixel. There is a method capable of reducing the largest variation of the primary scanning starting position to a 1/N pixel (N is a natural number), and this method will hereinafter be referred to as a "1/N method".
To carry out the "1/N method", on which the present invention is based, a delay element is required, and it is theoretically assumed that the delay element has no delay error between its taps, or no tap-to-tap delay error.
If there were no delay error between the taps of the delay element, the value N could be as large as desired, and the variation of the primary scanning starting position would theoretically be reduced to as small an extent as desired. However, since there is always a tap-to-tap delay error in an actual delay element, the value of N is at most 3 in the presence of such a tap-to-tap delay error.